1. Department of Veterinary Medical Chemistry, Swedish University of Agricultural Sciences, The Biomedical Centre, Box 575, S-751 23 Uppsala, Sweden, Department of Molecular Biology, Swedish University of Agricultural Sciences, The Biomedical Centre, Box 590, S-751 24 Uppsala, Sweden, and Department of Bioorganic Chemistry, Uppsala University, The Biomedical Centre, Box 581, S-751 23 Uppsala, Sweden